Get the type of the return value in C++

Question

Suppose we have a function f which returns a value of some unknown type (let\'s call it T) and takes a value of the type T as an argum

Answer 1

template 
struct return_type;

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
struct return_type { using type = R; };

template 
using return_type_t = typename return_type::type;

Test:

#include 

struct Functor
{
    int operator()(int i, int j) { return i + j; }
};

template 
struct A
{
    using T = return_type_t;

    T some_function(T some_arg) { return some_arg; }
};

int main()
{
    A a;
    static_assert(std::is_same::value, "!");
}

DEMO